The costs of maintaining active-duty and National Guard troops on the southern border have risen to $235 million to date, a Pentagon official said Tuesday.

About $132 million has been spent on the active-duty troops who began deploying to Texas, Arizona and California in early November, Vice Adm. Michael Gilday, operations director for the Joint Staff, told a House Armed Services Committee hearing.

National Guard troops who began deploying to the southern border on order of President Donald Trump last April have cost about $103 million through January, Gilday said.

He projected that the costs for maintaining National Guard troops on the border from last April through fiscal 2019, ending Sept. 30, would total about $550 million.

Gilday said it is "difficult to give you an estimate on active duty" because "the requirements are evolving and fluctuating" as troops back up Customs and Border Patrol, lay concertina wire between ports of entry, shore up existing barriers, aid in surveillance, provide medical support and perform other duties. Troops are barred under U.S. statutes from performing law enforcement functions.

About 5,900 active-duty troops initially deployed to the border. That number has come down to about 2,100, a Pentagon spokesman said Monday, though the border deployment has been extended to Sept. 30. There are currently about 2,350 National Guard troops on the border.

Both Gilday and John Rood, the Pentagon's under secretary for policy, declined to discuss the costs should Trump declare a national emergency to allow funds to be taken from the military construction budget to build extensions to existing segments of the wall.

The president has repeatedly said that he might declare a national emergency if Congress fails to provide $5.7 billion for the wall.

At an earlier, off-camera Pentagon briefing, Acting Defense Secretary Patrick Shanahan said that the Department of Homeland Security has already made requests for assistance from the Defense Department, which will likely require "several thousand" more active-duty troops to go to the border.

"More recently, DHS has asked us to support them in expanded concertina wire and expanded surveillance capability," Shanahan said. "We've responded with, 'Here's how many people it would it take.' "

When asked for a number, he said, "Several thousand. I'll kind of leave it at that."

Existing wall structures cover about 580 miles of the approximately 2,000-mile-long border, according to U.S. Customs and Border Patrol, but Rood said walls and other forms of barriers, such as concertina wire, extend for about 654 miles.

On CBS-TV's "Face the Nation" program Sunday, Mick Mulvaney, director of the White House Office of Management and Budget, said that the $5.7 billion sought by Trump would add about 283 miles of wall.

During the open session of a White House Cabinet meeting earlier this month, Shanahan said that active-duty troops are focusing on the "restoration of fences" and "building out additional mileage for the wall."

"The Army Corps of Engineers is dialed in on doing this cost effectively and with the right amount of urgency as to where we can build additional stand-up walls quickly and then get after the threat," he said.

At Tuesday's hearing, Rep. Adam Smith, D-Washington, the new HASC chairman, questioned whether illegal crossings had reached the point where the presence of active-duty troops is necessary.

"While border security is always a challenge, there's really not much evidence that right at the moment it is a crisis," Smith said.

Republicans on the committee backed up Trump's frequent declarations that there is a crisis, pointing to reports that more "caravans" of asylum seekers are headed north.

Rep. Mac Thornberry, R-Texas, the ranking Republican on the committee, said that the previous five administrations had authorized the use of the military to aid in border protection.

He said the Trump administration's actions are "in line with, consistent with, the sort of things that we have asked the military to do for a long, long time."

Funding for the wall -- and congressional Democrats' refusal to provide it -- was the main factor in the 35-day partial government shutdown that ended last Friday when Trump agreed to three more weeks of negotiations, with a Feb.15 deadline on his demand for wall funding.

The president has warned that the government could shut down again or he may declare a national emergency to take money out of the military construction budget if his demand is not met.